netlink: Disable duplicate address detection for configured IPv6 address

With default options, when we pass --config-net, the IPv6 address is
actually going to be recycled from the init namespace, so it is in
fact duplicated, but duplicate address detection has no way to find
out.

With a different configured address, that's not the case, but anyway
duplicate address detection will be unable to see this.

In both cases, we're wasting time for nothing.

Pass the IFA_F_NODAD flag as we configure globally scoped IPv6
addresses via netlink.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>

netlink.c

@@ -343,6 +343,9 @@ void nl_addr(int ns, unsigned int ifi, sa_family_t af,
 		if (af == AF_INET6) {
 			size_t rta_len = RTA_LENGTH(sizeof(req.set.a6.l));
 
+			/* By default, strictly speaking, it's duplicated */
+			req.ifa.ifa_flags = IFA_F_NODAD;
+
 			req.nlh.nlmsg_len = offsetof(struct req_t, set.a6)
 				+ sizeof(req.set.a6);